This invention relates to lasers and, more particularly, to lasers in which a population inversion is established by preferential removal of inner-shell d-electrons. The term "inner-shell" means any electron shell other than the outermost populated orbit for a given atom. The term "d-electron" corresponds to the standard quantum mechanical designation of the orbital angular momentum which includes the s, p, d, f . . . electrons having values of 0, 1, 2, 3 . . . .
Duguay and Rentzepis, Applied Physics Letters, Vol. 10, p. 350 (1967), first suggested theoretically the possibility of producing population inversions in the vacuum ultraviolet and X-ray spectral regions by X-ray photoionization of inner-shell p-electrons in atomic species such as Na and Cu. A detailed summary of this concept was given by Duguay in Laser Induced Fusion and X-Ray Laser Studies, edited by S. F. Jacobs et al, Addison-Wesley, Reading, Mass., p. 557 (1976), in which he pointed out the necessity for rapid pumping (e.g., about 50 psec risetime pulses for Na; femtoseconds for Cu) due to the short lifetime of the proposed upper laser levels. In addition, however, a population inversion is difficult to achieve because the lower laser level is an ion ground state which becomes rapidly filled due to electron collisional excitation of ground state neutral atoms. In order to reduce the requirement for rapid pumping, Mani et al, Journal of Applied Physics, Vol 47, p. 3099 (1976), proposed theoretically the use of He-like species such as Li.sup.+, whereby the removal of the inner-shell s-electron leaves the species in a metastable S-state, and the inversion is produced with respect to the ground state by rapid laser transfer of the population to a nearby P-state. (The upper case designations S, P, D . . . refer to the total orbital angular momentum and have values 0, 1, 2 . . . as for s, p, d.)
Harris et al in Laser Spectroscopy V, edited by A. R. W. McKellar et al, Springer-Verlag, New York, p. 437 (1981), proposed the transfer of energy from Li.sup.+ metastable levels to potential laser levels in neutral Li lying well above the ionization energy, by resonance pumping with an efficient, tunable anti-Stokes Raman source. In each of these proposals, high intensity broadband X-ray pumping sources, such as laser-produced plasmas, are used to rapidly produce the photoionized states. In order to alleviate the problems associated with producing a satisfactory X-ray filter to separate the X-ray source region from the region to be pumped, Harris et al in Laser Techniques for Extreme Ultraviolet Spectroscopy, edited by T. J. McIlrath et al, American Institute of Physics, p. 147 (1982), proposed theoretically that the X-ray plasma be produced within the volume of the Li vapor by focusing a 1.06 .mu.m laser onto a tantalum target within the vapor. Using this technique, Caro et al, Applied Physics Letters, Vol. 42, p. 9 (1983), were able to produce Li.sup.+ metastable densities as high as 6.times.10.sup.14 cm.sup.-3 a few mm away from a tantalum target, but they did not construct a working laser according to the Harris proposal. To date, there has been no report that the Harris laser has been built and successfully operated.